<style>
    div {
        height: 40px;
        width: 70%;
        background: black;
        margin: 50px auto;
        border-radius: 5px;
    }

    #rect {
        animation-name: rainbow;
        animation-duration: 4s;
    }

    @keyframes rainbow{
        0% {
            background-color: blue;
        }
        50% {
            background-color: green;
        }
        100% {
            background-color: yellow;
        }
    }


</style>
<div id="rect"></div>